The present invention relates to a safety device for an electrically controlled camera shutter mechanism, and it particularly pertains to such a device which ensures the forming of a slit between leading and trailing shutter blades.
Various types of slit shutters are already known, such as those having a pair of curtains which travel one after another in front of a film, or those having a pair of plates or disks which are respectively provided with an aperture and which are driven one after another such that their apertures overlap to form a slit for the film exposure. Electric control devices for those slit shutters are in general constructed to electromagnetically initiate a trailing shutter member, such as a curtain or a plate, in response to an electric signal generated by an electric circuit. This signal may be generated after a lapse of a time period from initiation of the leading shutter member in accordance with the scene brightness or a manually set shutter speed value. On the other hand, leading and trailing shutter members of slit shutters are generally overlapping in part with their light blocking portions to avoid unexpected exposure of the film.
Hence, it sometimes occurs that the trailing shutter member is initiated so early that the leading and trailing shutter members are driven without forming a slit, or with the formation of a slit so narrow as to cause a diffraction effect on the picture image.
On the other hand, when the electromagnetic device is so constructed that an electromagnet is previously energized to restrain the trailing shutter member at a cocked position thereof and de-energized in response to an electric signal to initiate the trailing shutter member, the shutter members can be nevertheless driven without forming a slit when the electric power source for the control circuit has been exhausted.
This no-slit shutter operation, i.e., travel of the leading and trailing shutter members without forming a slit, can generally not be recognized when the camera is operated, because the shutter is driven. One way to avoid this no-slit exposure is to warn, in an appropriate fashion, the photographer when such undesirable operation is expected to occur, such as due to too high a scene brightness or exhaustion of the source power. However, such a warning is not always noticed by the photographer.
There has been proposed another corrective measure which is to provide a delay means with the trailing shutter member or the control circuit. However, such delay means operates all the time, even at a moderate shutter speed, and it adds some delay time to the desired exposure time. This means that the shutter mechanism, including such delay means, always provides longer exposure times than is desired.
Although electronic systems have been devised in an attempt to overcome the aforementioned drawbacks, e.g., as in U.S. Pat. No. 3,683,767, they have not been found altogether satisfactory.
In U.S. Pat. No. 3,498,195 issued on Mar. 3, 1970, a shutter speed adjusting means is shown wherein an electronic circuit controls the release of a rear shutter screen after the front screen, after cocking, has been released. However, according to the particular structural setup described in this patent, the electronic circuit, and more specifically the integrating capacitor, is charged for operation only after a time lapse after the release of the front screen. Thus, if the power source therein is exhausted, the entire device is made inoperative.
Accordingly, the primary object of the present invention is to provide a shutter mechanism which is provided with a delay means for avoiding no-slit shutter operation but which does not have the drawbacks as mentioned above.
Another object of the present invention is to provide an electrically-controlled shutter mechanism wherein delay means for avoding no-slit shutter operation affects the exposure time only when necessary.
Still another object of the present invention is to provide an electrically-controlled slit shutter which can make exposure with minimum time when the electric power source has been exhausted.